In the past various laminated windshields, cockpit canopies, and other windows have been provided for aircraft. Such laminations can be made of rigid layers of glass or plastic, or combinations of glass and plastic sandwiched together by means of somewhat flexible interlayers such as polyurethane, polyvinyl butyral, silicone or the like. Most common plastics used are acrylates and polycarbonates.
Commonly such windshields are provided with electrically conductive circuit layers bonded between such glass and/or plastic layers. The circuits are used for electrically heating windows, draining static electricity from the surfaces, shielding from radio frequency interference, (RFI), electromagnetic interference (EMI), electromagnetic pulse (EMP), and the like.
One type of electrically conductive circuit layer provided in the past includes a thin film of conductive metal or metal oxide adhered on the inner surface of a rigid, outer windshield layer or face ply. Thin films of metal have also been deposited on thin plastic layers embedded in the laminated windshield for similar purposes. Another type of electrically conductive circuit layer employs a large number of more or less parallel sinuous metal wires embedded in the lamination.
Some disadvantages have been identified with respect to such electrically conductive circuit layers. The voltage and current relations required for electrical heating may be difficult to accommodate in some aircraft electrical systems. The total transparency of the window may be reduced in the case of a thin film conductive layer. A thin film layer may not be suitable on all substrates. For example, a transparent metal oxide thin layer is inappropriate for a plastic substrate. Fabrication of windows incorporating wire elements is costly and difficult because of the very small size wires required to maintain any semblance of optical quality. Further, thermal gradients created by heated wires embedded in a plastic interlayer away from the face ply of the window may result in gradients in index of refraction which can introduce blurring and distortion.
Another approach to providing a heater circuit on the surface of a glass panel in a window laminate is disclosed by Coale et al in U.S. Pat. No. 2,932,170. The electrical conductors disclosed by Coale, et al are applied to the glass surface by first masking the surface completely with a protective film. The pattern of the electrical conductors is then formed through the film by scribing or using photographic techniques to thereby expose only that portion of the glass that forms the desired conductor pattern. A metal such as silver, copper or gold is then applied to the exposed pattern on the glass surface by "conventional mirror-producing methods" or by vaporization. The protective film is then removed from remaining portions of the glass, leaving the conductive metal lines on the glass surface.
When a circuit that comprises electrically conductive lines is provided on a glass surface for heating a windshield that incorporates the circuit, it is important that the lines adhere to the surface. For example, if such lines do not adhere, non-uniform heating can result, which can distort vision through the windshield. Such vision distortion can be intolerable, for example, in aircraft windshields. Conductive lines that do not adhere well are also subject to damage in processing the face ply for incorporation in the window.
Although the disclosure of Coale, et al relates to forming electrically conductive lines on glass, aircraft windshields onto which such lines can be formed for use as a heater may be made entirely of plastic as a weight-saving measure.
A technique has been provided for forming a window having an electrically conductive circuit layer in U.S. patent application Ser. No. 529,975, filed September 7, 1983, which is a continuation-in-part of U.S. patent application Ser. No. 262,494, filed May 11, 1981. Both of these applications are assigned to the assignee of this application and are hereby incorporated by reference.
These applications describe a method for forming a plurality of spaced apart electrically conductive metal film runs or lines on the surface of a transparent substrate to be used as the outer layer or face ply of a heatable window. The inner surface of the outer layer has a transparent base coat applied and a strike coat of electrically conductive metal is deposited onto the base coat. A portion of the metal strike coat is masked so that unmasked portions of the strike coat comprise the pattern of electrically conductive metal film runs to be formed. An electrically conductive metal is electroplated onto the unmasked portions of the strike coat. The mask and portions of the strike coat onto which no metal was electroplated are then removed to leave a plurality of spaced apart electrically conductive metal film runs on the inner surface of the outer window layer. Flexible "bus bars" of metal braid are applied as leads to make electrical contact with the lines. The outer layer is laminated with other layers to sandwich the electrically conductive circuit layer within the window.
A base coat described in the aforementioned applications comprises a co-polymer reaction product of a methacrylic acid ester and an organofunctional silane. Another base coat comprises a composition that includes a urea-formaldehyde compound, an epoxy resin, and an organo-functional silane. The former base coat is cured by immersion in weak acid solution for about one to four hours. The latter base coat is cured by heating at about 300.degree. F. for about one to six hours.
The handling required until such a base coat can be cured affords an opportunity for particles and other contamination to adhere to the base coat and degrade the optical properties of the resultant window. It is also desirable to improve the reliability of adhesion between the base coat and strike coat. It is also desirable to provide a strike coat applicable to polycarbonate as well as acrylic substrates. It is desirable to improve the optical quality of the base coat for optimizing the optical quality of the composite window. For economy it is desirable that the base coat be applied easily, quickly, and reliably for economy of manufacture.